1. Field of the Invention
The present invention relates to an image reader, and particularly, to a manual scan type image reader for optically reading an image of a document (referred to as a document image hereinafter) etc. by a manual scan, converting the read image into image data, correcting the image data so as to faithfully reproduce the read image, and outputting the corrected image data to an external unit such as a personal computer.
2. Description of the Related Art
A conventional manual scan type image reader which is called an image scanner generally comprises a rotary encoder shown in FIG. 5 for detecting a position of a CCD image sensor for reading an image when the CCD image sensor is moved by a manual scan since a scan speed thereof is not constant.
Referring to FIG. 5, the rotary encoder is provided at the end portion of a scanner (not shown) comprising an image reading section composed of an optical system and a CCD image sensor. In the rotary encoder, a rubber roller 1 is rotated on a surface of a document to be read when the scanner is moved by the manual scan, and the turning force of the rubber roller 1 is transmitted to a slit circular plate 3 through a rubber belt 2. In the slit circular plate 3, there are formed plural slits radially and at equal spaces, and a transparent type photointerruptor 4 comprising a light emitting diode (referred to as an LED hereinafter) 4a and a phototransistor 4b is provided so as to oppose to the slit circular plates 3 through a slit body 5 for passing therethrough a beam of light projected from the LED 4a to the slit circular plate 3 and a beam of light reflected from the slit circular plate 3 to the phototransistor 4b. When a beam light emitted from the LED 4a of the transparent type photointerruptor 4 is projected through the slit body 5 onto the slit circular plate 3, the projected beam of light therefrom is reflected by the slit circular plate 3 and the reflected beam of light is projected through the slit body 5 onto the phototransistor 4b of the photointerruptor 4, and then, a detection signal for representing the position of the scanner which is moved by the manual scan is outputted to the CCD image sensor. On the other hand, the CCD image sensor reads the document image in synchronization to the detection signal, converts it into an electric image signal and outputs the electric image signal to an external unit such as a personal computer.
However, since the above-mentioned rotary encoder is constituted by mechanical rotation members such as the rubber roller 1 and the slit circular plate 3, the precision of the rotation ratio of the number of rotation per a unit time of the rubber roller 1 to that of the slit circular plate 3 is lowered if, for example, the rubber belt 2 for transmitting the turning force from the rubber roller 1 to the slit circular plate 3 expands and contracts. In this case, there is such a problem that image information of the document image can not correctly outputted to the personal computer.
Further, since a dimension of the rotary encoder itself is relatively large, it is difficult to miniaturize it, resulting in such a problem that the rotary encoder is not suitable for a handy type image scanner.